[Paper Review] Snowmass White Paper: Prospects of CP-violation measurements with the Higgs boson at future experiments

A. V. Gritsan, Bahl, H.+16 more|arXiv (Cornell University)|May 16, 2022

Dark Matter and Cosmic Phenomena7 citations

TL;DR

This Snowmass White Paper evaluates the prospects for measuring CP violation in Higgs boson interactions at future colliders, including photon, muon, hadron, electron-positron, and lepton-hadron machines. It projects sensitivity to CP-odd couplings via Higgs decay and production processes, showing that future experiments could probe CP-violating parameters at levels competitive with electric dipole moment (EDM) constraints—especially when multiple couplings are non-zero, where EDM limits become less stringent due to cancellations.

ABSTRACT

The search for CP violation in interactions of the Higgs boson with either fermions or bosons provides attractive reference measurements in the Particle Physics Community Planning Exercise (a.k.a. "Snowmass"). Benchmark measurements of CP violation provide a limited and well-defined set of parameters that could be tested at the proton, electron-positron, photon, and muon colliders, and compared to those achieved through study of virtual effects in electric dipole moment measurements. We review the current status of these CP-sensitive studies and provide projections to future measurements.

Motivation & Objective

• To evaluate the sensitivity of future colliders to CP-violating couplings of the Higgs boson with fermions and gauge bosons.
• To compare the projected reach of future collider experiments with existing constraints from electric dipole moment (EDM) measurements.
• To identify the most promising Higgs production and decay channels for CP-odd coupling measurements across different collider types.
• To quantify the impact of multiple non-zero CP-odd couplings, particularly the challenge of parameter space degeneracy due to EDM cancellations.

Proposed method

• Uses effective field theory (EFT) to parameterize CP-odd Higgs couplings via Wilson coefficients, relating them to observable decay and production rates.
• Applies the $f_{CP}^{X}$ formalism to express CP-violating effects in Higgs decays to $VV'$, $ff'$, and $gg$ final states.
• Projects sensitivity at future colliders by simulating signal rates and backgrounds for $H\to\gamma\gamma$, $HZ\gamma$, $HZZ$, $HWW$, $Ht\bar{t}$, and $H\to\tau^+\tau^-$ processes.
• Compares collider reach to EDM constraints by translating EFT coefficients into EDM predictions using hadronic and nuclear matrix elements.
• Performs marginalization over unconstrained linear combinations of couplings to assess allowed parameter space under EDM constraints.
• Uses the Rfit approach to vary nuclear matrix elements and assess uncertainty in EDM-based limits on $Hgg$, $Huu$, and $Hdd$ couplings.

Experimental results

Research questions

• RQ1 What is the projected sensitivity of future photon, muon, and electron-positron colliders to CP-odd Higgs couplings with gauge bosons and fermions?
• RQ2 How do EDM constraints compare to collider-based sensitivities when multiple CP-odd couplings are non-zero simultaneously?
• RQ3 To what extent do cancellations in EDM contributions allow large values of CP-odd couplings despite stringent EDM bounds?
• RQ4 How do uncertainties in nuclear matrix elements affect the interpretation of EDM constraints on Higgs CP-odd couplings?
• RQ5 Which Higgs production and decay channels offer the most sensitive probes of CP violation at future colliders?

Key findings

• Future colliders can probe CP-odd Higgs couplings with sensitivities competitive with current EDM constraints, especially for $H\gamma\gamma$, $HZ\gamma$, and $HZZ$ couplings.
• When only one CP-odd coupling is non-zero, EDM constraints are typically stronger than LHC limits by several orders of magnitude, except for $Hgg$, $Huu$, and $Hdd$ couplings.
• For multiple non-zero couplings, such as $f_{CP}^{H\gamma\gamma}$, $f_{CP}^{HZ\gamma}$, and $f_{CP}^{HZZ}$, a significant portion of parameter space remains allowed due to cancellations in EDM contributions.
• The neutron and mercury EDMs dominate constraints on $Hgg$, $Huu$, and $Hdd$ couplings, with uncertainties in nuclear matrix elements leading to a factor of $\sim10^3$ variation in limits when central vs. extreme values are used.
• EDM constraints on $f_{CP}^{H\gamma\gamma}$ and $f_{CP}^{H\tau\tau}$ are comparable to LHC sensitivity at 14 TeV, indicating that future colliders may surpass LHC in these channels.
• Even with EDM cancellations, collider measurements can exclude large regions of the parameter space, particularly when combined with multiple decay channels and production modes.